Solar power generation is becoming a progressively larger source of energy throughout the world. Solar farm collector systems utilize a plurality photovoltaic arrays (PV arrays) to convert solar energy incident on the PV arrays into DC power. The solar farm couples the DC output of the PV arrays to one or more DC to AC inverters in order to convert the DC output of the PV arrays into a suitable AC waveform that can be fed to the electrical grid.
In a typical solar farm collector system, a plurality of inverters providing suitable AC electrical outputs from one or more PV arrays are connected in parallel to at least one conductor or network of conductors. The collector system generally includes a plurality of transformers, with one or more of the transformers connected between each inverter and the at least one conductor. A substation transformer can be used to connect the solar farm collector system to the electrical grid.
Existing solar farm control techniques generally relate to voltage control and real and reactive power control, either at the individual inverters or at the point of common coupling for the system. Efficiency of the system, based on loss reduction, has generally not been considered in such control schemes.
Currents flowing in a solar farm collector system create losses due to the electrical resistance of the system. In addition, the collector system transformers have excitation losses that are independent of loading, but which increase with voltage to an exponential power typically greater than two, and often times close to three.
The load loss PLL (S,V), also known as the conduction loss or “copper” loss for a given solar farm complex power output S and voltage V, is related to the load loss PLL-rated at the rated power output Srated and rated (nominal) voltage Vrated as follows:
                                          P            LL                    ⁡                      (                          S              ,              V                        )                          =                                            (                                                V                  rated                                V                            )                        2                    ⁢                                    (                              S                                  S                  rated                                            )                        2                    ⁢                      P                          LL              ⁢                              -                            ⁢              rated                                                          Equation        ⁢                                  ⁢                  (          1          )                    
The no-load losses of the solar collector system transformers PNL(V), also called the excitation loss or “iron” loss, at any voltage V, is related to the no-load loss PNL-rated at rated Vrated as follows:
                                          P            NL                    ⁡                      (            V            )                          =                                            (                              V                                  V                  rated                                            )                        N                    ⁢                      P                          NL              ⁢                              -                            ⁢              rated                                                          Equation        ⁢                                  ⁢                  (          2          )                    where N is an empirically derived exponent unique to the magnetic design and materials of the transformers used in the collector system.
The total loss PLOSS(S,V) at any voltage and complex power level is the sum of Equation (1) and Equation (2), as described below as follows:
                                          P            LOSS                    ⁡                      (                          S              ,              V                        )                          =                                            (                                                V                  rated                                V                            )                        ⁢                                          (                                  S                                      S                    rated                                                  )                            2                        ⁢                          P                              LL                ⁢                                  -                                ⁢                rated                                              +                                                    (                                  V                                      V                    rated                                                  )                            N                        ⁢                          P                              NL                ⁢                                  -                                ⁢                rated                                                                        Equation        ⁢                                  ⁢                  (          3          )                    
The total loss, i.e. the sum of the “copper” losses and “iron” losses, can be reduced by controlling V. This can be accomplished, for instance, by differentiating Equation (3) with respect to V, and solving for the value of V where the first derivative is zero.
For typical parameters, FIG. 1 depicts the variation of total loss with voltage level for four different power levels. At 10% power, a low voltage provides reduced losses. At 30% power, a voltage near rated voltage provides reduced losses, and above this power level (e.g. at 50% and 100%), a high voltage provides reduced losses. The same information is shown in a 3-dimensional format in FIG. 2.
Certain loss reduction techniques are known, for instance, for use with wind farm collector systems. In solar farm systems, however, loss reduction concerns must also take into account the fact that the inverters are typically coupled relatively close together and that the solar farm system will have no power output during the night when solar energy is not available.
It would be possible to design a lower loss collector system by decreasing the collector system resistance, for instance by increasing conductor cross sectional area or by designing a higher voltage collector system. These alternatives, however, can require substantial equipment investment and costs such that the savings in reduced losses generally do not justify the equipment investment.
Thus, there is a need to provide a method and system for reduction of total losses of the solar farm collector system through distribution of reactive loads and voltage control, while maintaining essentially the same physical equipment and control structure for the system.